In a system where OFDM modulation is performed on a downlink, there is a need for measuring downlink channel quality by transmitting reference symbols, and performing frequency-dependent scheduling according to the channel quality measurements.
In a Long Term Evolution (LTE) system, it has been specified that the Orthogonal Frequency Division Multiple Access (OFDMA) mode is used for the downlink, and two frame structures, i.e. a Generic frame structure and an Alternative frame structure, are supported.
The Generic frame structure supports two types of duplex modes including a Frequency Division Duplex (FDD) mode and a Time Division Duplex (TDD) mode. In the Generic frame structure as shown in FIG. 1, a radio frame having a length of 10 ms is constituted of 10 subframes of the same length, and each of the subframes is constituted of two slots having the same length of Tslot of 0.5 ms. Each of the two slots may be constituted of 7 or 6 OFDM symbols, depending on the configured Cyclic Prefixes (CPs) of different lengths. A generic slot is constituted of 7 OFDM symbols if a generic CP (which is of a relatively small length) is used, and the generic slot is constituted of 6 OFDM symbols if an extended CP (which is of a relatively large length) is used.
Currently, a structure of common reference symbols has been determined for the Generic frame structure. The structure of the common reference symbols within a subframe when a generic CP is used is shown in FIG. 2a, and the structure of the common reference symbols within a subframe when an extended CP is used is shown in FIG. 2b. As shown, Ri denotes a common reference symbol transmitted by the ith transmission antenna, where i=1, . . . , 4, and one common reference symbol R1 is transmitted in the case of one transmission antenna, common reference symbols R1 and R2 are transmitted in the case of two transmission antennae, and common reference symbols R1, R2, R3 and R4 are transmitted in the case of four transmission antennae.
The Alternative frame structure supports the TDD mode and is compatible with a Time Division Synchronized Code Division Multiple Access (TD-SCDMA) system, as shown in FIG. 3. As shown, a radio frame having a length of 10 ms is constituted of two half-frames each having a length of 5 ms, and each of the half-frames is constituted of 7 generic slots (TS0˜TS6) and 3 special slots. The generic slots are used for transmitting data, and the 3 special slots are respectively a Downlink Pilot Time Slot (DwPTS) (which represents a downlink reference symbol channel in the LTE system and is used for transmitting system downlink synchronization information), an Uplink Pilot Time Slot (UpPTS) (which represents an uplink reference symbol channel in the LTE system and is used for transmitting uplink synchronization information for user access), and a Guard Period (GP) (which is a guard slot and is used for providing a time period for the transition from the downlink transmission slot to the uplink transmission slot). A generic slot is constituted of 9 or 8 OFDM symbols, depending on the configured CPs having different lengths. When the generic CP (which is of a relatively small length) is used, a generic slot is constituted of 9 OFDM symbols, and when the extended CP (which is of a relatively large length) is used, the generic slot is constituted of 8 OFDM symbols.
Currently, a structure of common reference symbols that has been determined for the Alternative frame structure is shown in each of FIGS. 4a and 4b. The structure of the common reference symbols in the case of the generic CP is shown in FIG. 4a, and the structure of the common reference symbols in the case of the extended CP is shown in FIG. 4b. Ri denotes a common reference symbol transmitted by the ith transmission antenna, where i=1, . . . , 4, and one common reference symbol R1 is transmitted in the case of one transmission antenna, common reference symbols R1 and R2 are transmitted in the case of two transmission antennae, and common reference symbols R1, R2, R3 and R4 are transmitted in the case of four transmission antennae.
It has been found that user-specific reference symbols are required to be introduced in addition to the common reference symbols if a closed-loop multi-antenna transmission technology such as beam-shaping is used. Therefore, there exists a need to transmit both the common reference symbols and the user-specific reference symbols.